Optical diffusers are widely employed, for instance, for a liquid crystal display to diffuse light emitted by a back light or a screen of a television set or a cinematic projection. The optical diffuser is used to diffuse a light efficiently and also sufficiently transmit a light therethrough. Further, an optical diffuser to be used in a color imaging system should not be colored.
A conventionally employed optical diffuser to be used for a back light of a liquid crystal display comprises a transparent polyester support and an optical diffusing layer which is coated on a surface of the support and comprises inorganic particles (matting agents) and a binder. The optical diffuser of this type cannot transmit a light at a satisfactory level and is not suitably employed in combination with a color imaging system. One improvement comprises use of a polycarbonate sheet (which has a transparency higher than the polyester) which has been subjected to embossing treatment. The embossed polycarbonate sheet is at present used as a back light diffuser for liquid crystal display. The diffuser of embossed sheet, however, has a disadvantage that a sheet should have large thickness for allowing satisfactory embossing. Further, the embossed polycarbonate diffuser is liable to receive static electricity, and therefore an antistatic agent is necessarily sprayed on the diffuser when it is installed within a liquid crystal display. Furthermore, the embossed polycarbonate sheet is disadvantageous in its high cost such as two or three times as much as the cost of the conventional diffuser.
As an alternative solution, increase of amount of light may be effective to increase the light transmission through an optical diffuser. However, the increase of light amount needs increase of high electric power as well as production cost, which is naturally unfavorable.
Recently, Nikkei Microdevice (issued in February of 1993) reports an optical diffuser employing a combination of an organic polymer binder and organic polymer particles. The reported optical diffuser employs polymer particles of large sizes, and a weight ratio of the polymer particles to the binder is low. According to a study performed by the present inventors, the reported optical diffuser cannot show satisfactory light transmission, and the luminance given by a back light equipped with the reported optical diffuser is low.